1. Field of the Invention
The present invention relates to a device and a method for outputting a charged particle beam to treat an affected area of the body by irradiating the affected part of the body with a charged particle beam of, for example, protons and carbon ions.
2. Description of the Related Art
A therapy performed by irradiating an affected part of the body of a cancer patient with a proton, carbon ion, or other charged particle beam (ion beam) is known. A device for outputting a charged particle beam, which is used for this therapy, includes a charged particle beam generator, a beam transport line, and an irradiation device. An ion beam accelerated by the charged particle beam generator reaches the irradiation device located in a rotating gantry through a first beam transport line and then a second beam transport line located in the rotating gantry. The ion beam is outputted from the irradiation device and then applied to an affected part of the patient's body. A synchrotron (circular accelerator) which includes, for example, means for circularly moving a charged particle beam along a circular path, means for resonating betatron vibration of the charged particle beam outside the stability limit of resonance, and an extracting deflector which extracts the charged particle beam from the circular path is known as a charged particle beam generator. An example is described in U.S. Pat. No. 5,363,008, which is hereby incorporated by reference.
In a therapy using an ion beam, for example, when irradiating the affected part of the body with a proton beam, characteristics in which a majority of the proton beam energy is released when proton stops, i.e., when a Bragg peak is formed, are utilized. In this therapy, proton is stopped near the affected part of the body through selection of incident energy of the proton beam so that the majority of the energy (absorbed dose) is applied only to cells of the affected part of the body.
Normally, the affected part of the body has a certain amount of thickness in the depth direction from the body surface of the patient, which is also a direction of ion beam propagation (hereinafter referred to simply as depth direction). In order to effectively irradiate the whole thickness of the affected part of the body in the depth direction with ion beam, it is necessary to control the energy of the ion beam so as to form a flat range of absorbed dose (spread-out Bragg peak hereinafter referred to as SOBP width), which is wide to some extent in the depth direction.
From such a viewpoint, a range modulation device (range modulation wheel hereinafter referred to as RMW) having a plurality of blades whose thickness varies stepwise in circumferential direction, arranged around a rotating shaft has conventionally been advocated to be used (for example, see REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 64 NUMBER 8 (AUGUST 1993), page 2077, FIG. 30; and PHYSICS IN MEDICINE AND BIOLOGY, VOLUME 48, NUMBER 17 (7 Sep. 2003)). The disclosures of both of these articles are hereby incorporated by reference. The plurality of blades is attached to the rotating shaft. The RMW forms an opening which penetrates between adjacent blades. For example, when the RMW is rotated with the opening positioned at an ion beam path (referred to as beam path), the opening and the blade alternately pass over the beam path. When the ion beam passes the opening, the beam energy does not attenuate and therefore a Bragg peak is produced at the deepest position in the body. When the ion beam passes a thicker portion of the blade, the energy of this ion beam attenuates more and accordingly a Bragg peak is formed at a portion nearer the body surface of the affected part. As the RMW rotates, the position of Bragg peak formation in the depth direction periodically varies. As a result, it is possible to obtain a flat Bragg peak which is comparatively wide in the depth direction of the affected part of the body. Furthermore, it is known that an SOBP width can also be formed with the use of a ridge filter (see REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 64 NUMBER 8 (AUGUST 1993), page 2078, FIG. 31).
The absorbed dose applied to the affected part of the body can be calculated by measuring a detection value proportional to the absorbed dose by the use of a dose monitor located more upstream than the patient on the axis of ion beam propagation and using a coefficient for converting the detection value into an actual absorbed dose value. It has been proposed that a conversion factor between the detection value by the dose monitor and the absorbed dose value applied to the actual affected part of the body has a correlation between the depth reached by the beam and the SOBP width (for example, in PHYSICS IN MEDICINE AND BIOLOGY VOLUME 48 NUMBER 17 (7 Sep. 2003)).